A laser diode is a semiconductor device which emits light of substantially a single wavelength. The light from a laser diode can be focused to a spot with a diameter comparable to the light wavelength. The laser diode belongs to the same family of semiconductor devices as the LED (light emitting diode). However, the light from an LED has a broader spectrum of wavelengths and thus cannot be focused as sharply as a laser diode. The configuration and composition of the laser diode determines its wavelength, expected lifetime and light guiding mechanism.
Semiconductor laser diodes are generally mounted on a substrate or structure which provides the electrical, thermal, and spatial needs of the laser diode for the intended application. The combination of a substrate or structure with a laser diode is commonly referred to as a laser assembly. Examples of laser assemblies are shown in U.S. Pat. Nos. 3,257,626, 3,293,513, 3,457,468, 3,479,613, and 4,483,480.
Most commercial laser assemblies contain a photodetector behind the laser to monitor the level of light emitted by the laser diode. The monitoring photodetector provides a signal which is used to maintain a constant laser output. It has been noted in some applications that when a small amount of light emitted from the laser is fed back into the laser diode, it causes a change in the laser output power which can be detected by the monitoring photodetector.
Many commercial applications use a second photodetector in front of and below the laser diode to monitor laser light reflected off of an object located in front of the laser diode. The second photodetector provides a signal which can be used to determine the reflective properties of the illuminated portion of the object such as optical storage media.
FIGS. 1a and 1b show front and side views, respectively, of a typical prior art laser assembly 10. A semiconductor laser diode 12 and a tilted rear photodetector 13 are mounted on a metal heatsink 14 which, in turn, is mounted on a header 15. The header has three output pins 16, 18, and 20 for external connections. The photodetector is slightly tilted and is located behind the laser diode as shown in the side view of FIG. 1b. This tilt of the photodetector is provided to prevent laser light, which is reflected off of the photodetector, from being fed back into the laser diode.
A wire 22 connects the top or anode of the laser diode to a flat surface on pin 16 that is parallel to the top of the laser diode. A wire 24 connects the front or anode of the photodetector to a beveled end of pin 18. The bottom or cathode of the laser diode is electrically coupled to pin 20 through the heatsink and the header 15. The cathode of the photodetector is also electrically coupled to pin 20 through the header. This provides a three pin laser assembly which is typically packaged in a standard T05 can. Because the cathode of the laser diode shares pin 20 with the cathode of the photodetector, the laser assembly is sometimes called a common cathode device.
Because wires must be bonded on a horizontal surface due to gravitational effects, the laser assembly, including the header and pins, must be placed on its side as shown in FIG. 1b during bonding of wire 22 from the anode of the laser diode to pin 16. After wire 22 is bonded, the laser assembly is then rotated approximately 90 degrees to the orientation shown in FIG. 1(a) for bonding wire 24 from the anode of the rear detector to pin 18. As a result, the bonding process is made more complex because the header and pins must be rotated.
In an alternative assembly, the laser diode is attached to the heatsink with the anode of the laser diode in contact with the heatsink and the cathode of the laser diode in contact with wire 22. In this alternative assembly, the cathode of the photodetector shares pin 20 with the anode of the laser diode. A variation to the three pin devices is a four pin device which uses a silicon submount to electrically isolate the cathode of the laser diode from the heatsink.
In any of the above-described prior art assemblies, the header and pins must be rotated during the bonding process.
FIG. 2a shows the electrical circuit of the common cathode laser assembly shown in FIGS. 1a and 1b. Corresponding elements to FIGS. 1a and 1b have primed corresponding numbers. The anode of laser diode 12' is electrically coupled to pin 16'. The anode of photodetector 13' is electrically coupled to pin 18'. The cathodes of laser diode 12' and photodetector 13' share pin 20'.
FIG. 2b shows the electrical circuit of the alternative to the laser assembly shown in FIGS. 1a and 1b. Corresponding elements to FIGS. 1a and 1b have primed corresponding numbers. The cathode of laser diode 12' is electrically coupled to pin 16'. The anode of photodetector 13' is electrically coupled to pin 18'. The anode of the laser diode and the cathode of the photodetector are electrically coupled to pin 20'.
FIG. 3 shows a prior art laser assembly 30 as disclosed in Japanese Patent Application Number 59-117279(A). This laser assembly uses rear facet detection of the laser output to maintain a constant laser output. A laser diode 32 is attached to a silicon submount 34 which, in turn, is mounted on a copper heatsink 36 with a plurality of output pins 38. The silicon submount is also a photodetector for monitoring the rear facet emission of the laser diode.
The upper surface of the silicon submount next to the heatsink is generally the cathode of the photodetector and is electrically coupled to the copper heatsink by physical contact. A first pole of the laser diode is generally connected to the cathode of the photodetector 34 by a wire. The anode of the photodetector and a second pole of the laser diode are electrically coupled to the pins by separate wires.
Given the geometry of the laser assembly, the wires are first attached to the laser diode and the photodetector, the laser assembly is rotated 90 degrees, and then the wires are attached to the pins. Again, the header and pins are rotated during the bonding process.
FIG. 4 shows a front view of a laser assembly 40 as disclosed in U.S. Pat. No. 4,757,197. A semiconductor laser diode 42, a tilted rear photodetector 44 (similar to tilted rear photodetector 13 of FIGS. 1a and 1b), and a forward four quadrant photodetector 46 are mounted on a heatsink 48. The heatsink, in turn, is mounted on a header 50 which has a plurality of output pins 52a-h. The tilted rear photodetector is used to detect rear facet emissions of the laser diode in order to maintain a constant laser output. The forward four quadrant photodetector detects the laser light from the laser diode that has reflected off of an object located in front of the laser diode.
A wire 54 electronically couples the top, or anode, of semiconductor laser 42 to pin 52a. Four wires 56a-d connect a first pole (preferably the anode) of the four quadrants of the four quadrant photodetector to pins 52b, 52d, 52e, and 52f, respectively. Wire 58 connects a common second pole (preferably the cathode) of the four quadrant photodetector to pin 52c. Wire 60 connects the anode of the rear photodetector to pin 52h. The cathode of the laser diode and the rear photodetector are electrically coupled to pin 52g through the heatsink and wire 62.
The laser assembly must be placed on its side while bonding wire 54 from the anode of the laser diode to pin 52a and wire 62 from the cathode of the laser diode to pin 52g. In addition, wires 56a-d and 58 are bonded from the poles of the four quadrant photodetector to the output pins. Furthermore, wire 60 is bonded from the anode of the rear photodetector to pin 52h. As with other prior art, the laser assembly, including the header and pins, must be rotated during the bonding process.
The prior art laser assembly 10 shown in FIG. 1 has many applications, one of which is in the optical head assembly 70 shown in FIG. 5. The laser assembly contains a laser diode to provide a light beam 72 which, after passing through a grating 74, a beam-splitter 76 and a collimating lens 78, is focused on an information medium 80 at spot 81 by an objective lens 82. The objective lens is supported by a focus and tracing actuator 83. When the light beam is reflected off the information medium, part of the beam is reflected by the beam splitter through a cylindrical lens 83 to a photodetector 84. The signal recorded on the information medium can be read out from photodetector 84.
FIG. 6 shows an optical head assembly 85 using the prior art laser and detector assembly 40 shown in FIG. 4. The laser and detector assembly radiates a laser beam 86 to a collimating lens 87. The collimated beam passes through a hologram lens 88 to an objective lens 90 and focus on an information medium 91 at spot 92. The objective lens is supported by a focus and tracking actuator 93. When the light beam is reflected off the information medium, part of the beam is diffracted by the hologram lens and focuses on the detector part of the laser and detector assembly. A recorded signal on the information medium can be recovered from the signal coming out from the detector.